1. Field of the Invention
The present invention relates to a communication system in which wireless communication is performed between a control device mounted in a vehicle and a plurality of portable devices, that is, to the communication system such as a keyless entry system in which a door is locked or unlocked based on wireless communication between the in-vehicle control device and the portable device, for example.
2. Description of the Related Art
A keyless entry system is known in the related art (for example, see Japanese Unexamined Patent Application Publication No. 2010-219628). In the keyless entry system, an operation for a vehicle, such as locking or unlocking of a door of the vehicle, and a start of an engine is performed based on wireless communication between an in-vehicle control device and a portable device. The portable device receives a request signal from the in-vehicle control device and transmits a response signal, and thereby authentication processing is performed. In addition, a passive•keyless entry system has also been widely used. In the passive•keyless entry system, an operation for a vehicle, such as unlocking of a door is permitted in a case where it is determined that a portable device which has transmitted a request signal is a portable device which has been registered in advance on the in-vehicle control device, and thus the authentication succeeds.
In a general passive•keyless entry system, a plurality of antennae is disposed at positions of a vehicle. In a case where communication in a passive mode is started, firstly, an in-vehicle control device selects one antenna from the plurality of antennae, and a signal (LF signal) in a LF band is transmitted from the selected antenna. The transmitted LF signal includes a signal for causing a start of the portable device or data for authentication. If the portable device receives the LF signal, a signal (RF signal) in a RF band, which includes data for authentication is transmitted to the in-vehicle control device from the portable device. The in-vehicle control device performs authentication of whether or not the portable device is a portable device which has been registered in advance, based on the RF signal transmitted from the portable device. If the portable device passes the authentication, a control of the vehicle, such as unlocking of the door, is permitted. In a case where receiving the RF signal from the portable device is not possible, or in a case where the received RF signal does not pass the authentication, the in-vehicle control device selects another antenna and transmits the LF signal again.
However, in a case where a plurality of portable devices is registered in the in-vehicle control device, if RF signals for responding to the LF signal are transmitted from the plurality of portable devices so as to overlap each other in time domain, normal reception of the RF signals by the in-vehicle control device may not be possible. In order to avoid such circumstances, generally, a method in which transmission periods of the RF signals for the portable devices are assigned in advance so as not to overlap each other, and the RF signals are transmitted from the portable device in order has been employed.
However, this method has a problem in that as the number of registered portable devices is increased, a period of time required for determination of a state (non-existence state of the portable device) in which there is no portable device near to an antenna becomes longer. That is, if reception processing is continuously performed until the last transmission period for all registered portable devices and thus it is finally confirmed that receiving the RF signal is not possible, conclusive determination of “the non-existence state of the portable device” is impossible. As a result, in “the non-existence state of the portable device”, a period of time from when the LF signal is transmitted through one antenna until the one antenna is switched to another antenna and the LF signal is retransmitted becomes longer. For example, there is a case where because the portable device is in the rear portion of the vehicle, receiving the RF signal through an antenna in the front portion of the vehicle is not possible, and thus the antenna is switched to an antenna in the rear portion of the vehicle, so as to transmit the LF signal. In this case, the door is not rapidly unlocked, and thus, convenience is lowered.
In the control system disclosed in Japanese Unexamined Patent Application Publication No. 2010-219628, firstly, a response signal (RF signal) to a request signal (LF signal) transmitted from an in-vehicle system is transmitted from a plurality of portable devices at a common timing. Then, a response signal (RF signal) to the request signal (LF signal) is transmitted from each of the portable devices at an individual timing thereof. Thus, in a case where only one portable device is near to an antenna, the in-vehicle system receives the first response signal which has been transmitted at the common timing normally. Accordingly, even when a plurality of registered portable devices is provided, the response signal of the portable device is received normally by the in-vehicle system during the shortest period of time. In a case where a plurality of portable devices is provided, a response signal is transmitted from each of the portable devices at the individual timing thereof, and thus the response signal from each of the portable devices is received normally by the in-vehicle system.
However, even in the control system disclosed in Japanese Unexamined Patent Application Publication No. 2010-219628, the above-described problem in that as the number of registered portable devices is increased, the period for determination of “the non-existence state of the portable device” becomes longer is not solved. That is, if response signals have been continuously received until an individual timing of the last portable device among all registered portable devices, conclusive determination of “the non-existence state of the portable device” is impossible. Thus, in a case where an antenna is switched in order to receive a response signal from a portable device which is in a vehicle, the control of the vehicle, such as unlocking of a door is delayed longer as the number of registered portable devices is increased.